Research into morphology and phase structure in the surface of Al-Si alloy modified by yttrium oxide

BULLETIN OF THE POLISH ACADEMY OF SCIENCES TECHNICAL SCIENCES, Vol. 67, No. 2, 2019 DOI: 10.24425/bpas.2019.12428 Research into morphology and phase structure in the surface of Al-Si alloy modified by yttrium oxide D. ZAGULYAEV 1, S. KONOVALOV 2 *, V. GROMOV 1, A. MELNIKOV 2, and V. SHLYAROV 1 1 2 Department of Natural Sciences, Siberian State Industrial University, Novokuznetsk, Russia Department of Metals Technology and Aviation Materials, Samara National Research University, Samara, Russia Abstract. Using methods of physical material studies (scanning electron microscopy and micro X-ray spectral analysis), a study was carried out with focus on alteration of structure and phase composition in surface layers of Al-Si alloy (silumin АК10М2N) treated in electroexplosive alloying with a multiphase plasma jet formed in the process of aluminum foil explosion and carrying particles of Y2 O3 weighted powder portion. It was revealed that a porous surface layer with non-homogeneously distributed alloying elements (silicon, yttrium) in it is formed in any conditions of electroexplosive alloying of silumin. Thickness of the modified layer is different, varying 50 to 160 µm, depending on the zone to be examined. The modified surface consists basically of Al, Si and Y. Yttrium in the modified layer is thought to be an indirect evidence of better physical and mechanical properties of the surface layer in comparison with the base material. Key words: silumin, microstructure, electroexplosive alloying, yttrium oxide. 1. Introduction To date, engineering and technology are challenged with the need to develop and manufacture new materials with satisfactory operational properties. Since the surface of machine parts tends to get fractured while in operation, efforts are made to elaborate methods for strengthening and protection of the surface involving modification of properties in material surface or coating. This is a technique of surface treatment by concentrated energy flows, e.g. laser emission [1‒3], electron beams [4, 5] and plasma [6, 7]. It is characterized by pulse and local effect on the surface; that is supposed to be a significant economic advantage over stationary techniques. Furthermore, a number of treatment parameters and their combinations can be set, so new structure and phase states are possible in formed surface layers of materials with advanced properties. This study focuses on formation of coatings by means of electroexplosive alloying. This technology enhances strength, durometric and tribological properties of the modified material. Strengthening is possible due to formation of coatings with fine-dispersed phases in a viscous metallic matrix. As a consequence of electroexplosive alloying, physical and mechanical properties, such as micro-hardness, impact resistance, durability, as well as frictional characteristics can be changed significantly. Industrial Al-Si alloy – silumin АК10М2N was used as a material for research. It was a well-weighted decision because aluminum and aluminum-based alloys show a number of unique properties and are widely applied in mechanical engineering, *e-mail: Manuscript submitted 2018-08-23, revised 2018-09-25, initially accepted for publication 2018-09-30, published in April 2019. Bull. Pol. Ac.: Tech. 67(2) 2019 aircraft and car building. To modify the surface yttrium oxide powder ( Y2O3) was used in the study. This powder is broadly applied in the present day material studies for the development of advanced oxidation-resistant materials and manufacturing of highly efficient alloys [8, 9]. Up to now, far little attention has been paid to interrelation of chemical composition, structure and mechanical properties. Improvement of mechanical properties via various modification techniques hasn’t found sufficient reasoning yet. Different research groups all over the world have been working on this issue. These studies are focused on the influence of thermal treatment of aluminum and aluminum-based alloys, deformations in different conditions, alloying, and plasma treatment on modification of properties in the initial material [10‒17]. 2. Material and experiment methods Al-Si alloy АК10М2N was used as a material for research. Using the method of X-ray spectral analysis, chemical composition of analyzed samples was defined with the help of an energy-dispersion detector of micro X-ray spectral analysis INCAx-act; as revealed, main elements in the composition are Al – 84.88% and Si – 11.10%. Principal alloying elements are Cu – 2.19%, Ni – 0.92%, and Mg – 0.58%. Samples to be analyzed were 20£20£10 mm3 and oriented perpendicular to the axis of a plasma jet. Electroexplosive alloying was carried out using a laboratory pulse discharge unit EVU 60/10M [18]. Capacity storage of unit EVU 60/10M is discharged with a current density of approximately 1010 А/m2 on the conductor to be exploded, which was fixed on the electrodes of a coaxial end-type plasma accelerator. Multiphase plasma, generating in electroexplosive destruction of a conductor, gets a shape of a jet, which influences on the surface. 173 D. Zagulyaev, S. Konovalov, V. Gromov, A. Melnikov, and V. Shlyarov As for construction, electroexplosive unit EVU 60/10M consists of three main parts (Fig. 1): charger – 2, comprising an autotransformer, a step-up transformer and a rectifier; capacity storage – 3; plasma accelerator – 4. The unit is operated manually – it is charged and discharged via pressing particular buttons on the remote control – 1. A process chamber, where electroexplosive alloying is conducted, is connected with a prevacuum pump (5), controlled with the remote (1). and reflecting from it causes formation of a shock-compressed layer with high temperature and pressure. The surface is heated up to the temperature of melting and above it over a short time of pulse plasma impact. Aluminum foils were used as a material of exploded conductors, and Y2O3 – as a weighted powder portion. Surface treatment of silumin was carried out in six treatment conditions, differing in the voltage of discharge and weights of a weighted powder portion. The conditions are given in Table 1. Table 1 Conditions of electroexplosive alloying Fig. 1. Structural layout of electroexplosive unit EVU 60/10M. 1 – remote control, 2 – charger, 3 – capacity storage, 4 – plasma accelerator and process chamber, 5 – prevacuum pump When a capacity storage discharges, high density electrical current flows through electrodes (2) and conductor (6) (Al foil) to be exploded, causing, this way, its explosion (Fig. 2). The products of explosion are drawn into the vacuum process chamber (3) (residual pressure 100 Pа), carrying particles of the weighted powder portion ( Y2O3 is used for the purpose of research). The sample is placed in the vacuum process chamber at various distances from the nozzle and fixed with holders of samples (4). The products of electric explosion are a multiphase system, comprising both (...truncated)